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Always tired? Kids never got enough shuteye, even back in grandpa's day.

That's according to a century's worth of expert advice and sleep studies, which a team of researchers has now distilled into a brief report in the journal Pediatrics.

"There is a common belief that children are not getting enough sleep and that children's total sleep time has been declining," Lisa Anne Matricciani of the University of South Australia in Adelaide and colleagues write.

And while it's true that kids aren't getting as much bedtime now as they were in the late 1800s, that doesn't mean experts weren't worried back then, too.

In fact, as the Australian researchers combed through older and older literature, the recommended sleep time was always a good half-hour higher than what kids, or their parents, said they got.

"No matter how much sleep children are getting, it has always been assumed that they need more," the team says.

So why are the standards slipping?

According to Matricciani and colleagues, there just isn't any good science on which to base recommendations. They went through 32 sets of sleep advice, and only one provided any reasoning for its guidance: the actual sleep of 500 healthy kids.

No science behind numbers

Today, the US National Institutes of Health says adults commonly need between eight and eight and a half hours of sleep, whereas newborns should get 16 to 18 hours a day.

Children fall in between, with preschoolers needing 11 to 12 hours of slumber and older kids and adolescents 10 hours.

Those standards are based on how long people sleep when they're not interrupted. But one expert says there is still little ironclad science behind the numbers.

"We need to do due diligence and do the nitty-gritty effort of measuring sleep in a large group of the population to find out what's normal," said Dr David Gozal, an expert in childhood sleep problems at the University of Chicago. "That has never been done."

"If you don't know what's normal because you haven't measured it, then your recommendations are going to reflect what you believe is normal, although that's not necessarily correct."

That also explains why expert recommendations have been shifting downward over time, says Gozal, because physicians are influenced by changing societal expectations the same as everybody else.

"It only reflects the nature of our parental expectations," he says.

Numbers keep falling

Based on 218 articles that contained self- or parent-reported sleep for children, the Australian researchers estimate that kids' actual sleep duration fell by 73 minutes over a century.

By comparison, recommended sleep times dropped by 71 minutes, but remained 37 minutes above the estimates of real sleep.

Like other researchers, Gozal blames our shorter nights on the accelerated pace of modern society with its 24/7 demands on parents and kids alike.

"The concern that kids aren't sleeping enough is real," he says. "More than 80 per cent of parents need to wake up their kids, indicating that their kids don't get enough sleep."

Although it's next to impossible to prove conclusively that diminished shuteye is taking a toll on our health, several studies have linked it to a plethora of ailments - from obesity in American kids to attention problems in Korean high school students to heart disease around the globe.

"We are in fact reducing the amount of sleep as a society, and that is translating, at least in my mind, to an increased risk of many diseases," says Gozal.

So how do you know when your kid has had enough sleep?

"You will know that your child is sleeping enough if they wake up on their own rather than being awoken," says Gozal. "If they don't get up on time, make them go to bed earlier."